Observations of Coherence in Bacterial Reaction Centers Using Two-Dimensional Electronic Spectroscopy

Bacterial reaction centers (BRC) are photosynthetic proteins which perform electronic energy and charge transfer at ultrafast timescales (fs - ps) with near unity quantum efficiency in wild type proteins. Several recent observations of coherences in BRCs and other photosynthetic proteins have proposed that coherent superposition states are partly responsible for the impressive functionality of these systems. In attempting to answer this question, conflicting origin assignments of the coherences have been proposed. In order to develop our understanding of coherences we have performed Two-dimensional electronic spectroscopy on BRCs from purple bacteria Rh. capsulatus and its predominant pigment Bacteriochlorophyll a (BChla). We are able to identify strong intramolecular vibrational contributions in both systems as well as signs of vibronic coherence in the BRC. We additionally resolve the weak upper-excitonic peak of the strongly coupled special pair in the BRC, made visible due to resonance of vibrational modes with excitonic energy gaps. We simulate the signatures of vibronic coherence using a reduced BRC model. These results should inform future efforts to model electronic structure of the BRC.